Burners utilize combustion processes which involve the mixing of fuel and air and generating energy, typically in the form of heat, from the combustion of this mixture. The current method of increasing the volumetric heat release of the combustion is to increase the gross fuel input. This technique extends the flame length which is a drawback for burners with fixed length enclosures, such as radiant tube burners or gas turbine combustors.
Another method to increase the volumetric heat release is to increase the rate of mixing between the fuel flow and the oxidant flow. Typical devices which increase the rate of mixing include baffles, which are a series of perforated plates placed in the flow paths to introduce turbulence, and swirlers, which produce a spiralling motion in the body of the combined flow. Both of these devices are effective but produce significant momentum losses in the flow which reduce efficiency. Additionally, neither method is amenable to manipulation for the purpose of modulating the rate of mixing in order to control the heat release from the combustion.
A device that has been found to increase mixing without the accompanying momentum loss is a convoluted trailing edge placed between the flows to be mixed. The convoluted edge produces a series of large scale streamwise vortices of alternating rotation, generating an exchange of fluid between the flows, which rapidly break down into random turbulence. The alternating rotation of the vortices imparts no net angular momentum to the flow and tests have shown that the mixing is accomplished without significant momentum losses. This technique is described more fully in AIAA Paper No. 89-0619 Flame Propagation Enhancement Through Streamwise Vorticity Stirring, AIAA 27th Aerospace Sciences Meeting (Jan. 9-12, 1989).
Convoluted trailing edges have found many applications in areas where increasing the rate of mixing between flows is beneficial. In U.S. Pat. No. 4,835,961 fixed convoluted trailing edges were placed between a primary high energy flow and a low energy secondary flow in order to improve the pumping efficiency and thrust of an ejector pump. In U.S. Pat. No. 4,815,531 fixed convoluted trailing edges are used to improve the heat transfer in a flow over a heat source by increasing the rate of mixing within the flow and by minimizing the build-up of a thermal boundary layer. In U.S. Pat. No. 3,937,008 a fixed, convoluted, and canted trigger mechanism was suggested to promote rapid mixing and combustion. The convolutions were canted relative to the direction of flow in order to produce a swirl. The cited result is a low NO.sub.x emission combustion and an axially shorter combustion chamber.
Although the many advantages of using convoluted trailing edge devices to increase mixing are well documented, there is no prior art describing the use of such a device to modulate the rate of mixing.